168 THE ROYAL SOCIETY OF CANADA 



a little mercury. The mercury was gradually heated and a series of 

 photographs was taken with a small Hilger quartz spectrograph. A 

 reproduction of one of these photographs is shown in Fig. 1. The 

 upper spectrum is that of the mercury arc alone, the second is that 

 obtained when the quartz tube was moderately heated and the third 

 is that obtained when the mercury vapour density was considerably 

 higher. The asymmetrical character of the absorption band at 

 X = 2536-72 A°.U. is clearly brought out by the photograph. 



In the second experiments a photograph was first taken of the 

 spark spectrum of mercury in air in a manner already described in a 

 previous communication by one of us.^ 



Photographs were also taken of the spectrum of the light from 

 the spark between terminals of cadmium in air after it passed through 

 the mercury vapour in the exhausted quartz tube mentioned above. 

 These were taken with gradually increasing vapour density and are 

 shown in Fig. 2. In this photograph the mercury spectrum is shown 

 at the top well down into the ultra-violet and the strong lines at 

 A= 1942-1 A°.U. and A= 1849-6 A°.U. are clear and distinct. 

 The succeeding four spectra show that even with small vapour density 

 the absorption was such as to cut off the light of wave-lengths in the 

 region of A = 1942 A°.U. and A = 1849-6 A°.U. In the second last 

 spectrum, absorption at A = 2536-72 A°.U. can just be detected but 

 in the last one it is well marked. The absorption band at A = 2338 

 A°.U. also comes out in this spectrum and that at A = 1849 A°.U. has 

 widened out so that on the side of longer wave-lengths it has reached 

 A = 2144-0 A°.U. From the general appearance of the photograph 

 it will be seen that the absorption at A = 1849-6 A°.U. develops 

 symmetrically with increasing vapOur density. This photograph 

 also shows that light of wave-lengths near to A = 1849-6 A°.U. was 

 the most strongly absorbed by mercury vapour. That in the neigh- 

 bourhood of A = 2536-72 A°.U. came next, while high vapour den- 

 sities were required to bring out the absorption at A = 2338 A°.U. 



In the third experiment a large Hilger quartz spectrograph was 

 used. With this instrument the arc spectrum of mercury from a 

 quartz lamp was first taken, then the spark spectrum between alumi- 

 nium terminals in water after the manner devised by Henri^ and the 

 the spectrum of the light from the spark between these aluminium 

 terminals in water after it had passed through a heated clear fused 

 quartz evacuated tube containing mercury vapour of high density. 

 These three photographs are shown in Fig. 3. The spark from 



iMcLennan. Proc. Roy. Soc. A. Vol. 91. p. 26, 1914. 

 «Henri. Phys. Zeit. No. 12. p. 516. June 15th, 1913. 



